Hair straightening or relaxing has become increasingly popular in view of hair styles which require relatively or perfectly straight hair. Several types of hair straightening products are available, but sodium hydroxide is generally judged the most effective agent for substantially permanently straightening kinky-curly negroid hair. Until now, however, relatively high concentrations of greater than about 2.5 weight percent sodium hydroxide were required resulting in decreased hair condition benefits and increased skin irritation.
Hair straightening or relaxer products most commonly used in salons and in the home contain as the sole active hair-straightening agent, either a strong alkali, such as sodium hydroxide or guanidine hydroxide; a sulfite, typically an ammoniacal mixture of bisulfite and sulfite; or a thiol compound, typically ammonium thioglycolate. All of these types of products exert their primary reducing effect by breaking the cystine disulfide bonds present in hair keratin, referred to as hair keratin-disulfide bonds. The chemical action of alkali-based straightening, sulfite-based straightening and thiol-based straightening is known to differ to varying extents, with alkali, especially, producing additional stable crosslinks in the hair that are not normally present in virgin hair.
The majority of hair-straightening kits sold for home use are based on the sulfite or thioglycolate straighteners. However, these have several inherent disadvantages. One major disadvantage is the highly offensive odor of the thioglycolate solutions and of the thiol-reduced hair. Another disadvantage is that thiol-based straightening requires the use of an oxidizing neutralizer, such as hydrogen peroxide, to chemically relink the hair keratin-disulfide bonds and stop the straightening process quickly. Since the thiol-reduced hair is in an alkaline state, any excess neutralizer must also be removed to avoid bleaching the natural color of the hair. Sulfite-based straighteners have similar disadvantages. For example, sulfite-containing solutions can deteriorate gradually and release offensive odors of sulfur dioxide. Sulfite-reduced hair also must be neutralized by bringing the hair to an alkaline pH to reverse the keratin-sulfite reaction and chemically re-link the hair keratin-disulfide bonds.
On the other hand, strong alkalis, such as sodium hydroxide and guanidine hydroxide, have several advantages over the sulfite or thioglycolate agents. These alkalis do not have a highly objectionable odor or cause such an odor on reducing the hair. Alkali-straightened hair is treated at a highly alkaline pH of between about 12 and about 14. At that alkalinity, alkalis are known to form stable irreversible cross-links of lanthionine and lysinolanine in the reduced hair making a chemical relinking step unnecessary. Thus, the only step required following an alkali-based straightening process is to remove substantially all excess alkaline solution to avoid and minimize damage to the hair protein or skin. For this purpose, an acidic shampoo is usually used to neutralize residual alkali on and remove it from the hair and scalp. Guanidine hydroxide, unlike sodium hydroxide, is not chemically stable in solution for any practical storage period. Consequently, it must be freshly prepared prior to using (usually within 24 hours). For this reason, guanidine hydroxide-based straighteners are supplied as a two-component package. Thus, despite its causticity, sodium hydroxide-based hair straightening or relaxer kits, which became available around 1958, are still popular and widely used in professional shops. The sodium hydroxide-based relaxer has also continuously gained popularity for home use since its introduction to the retail market in 1971.
The main advantages of a sodium hydroxide straightener are relatively fast processing times and good straightening of naturally kinky-curly negroid hair. Additionally, the straightening effect is more permanent; i.e., less likely to revert to a curly state after shampooing and wearing than is hair straightened with other straighteners.
The principal disadvantages of sodium hydroxide-based hair straighteners, of course, is their causticity which can adversely affect hair condition, leaving it in a brittle state and harsh to the touch. Additionally, prolonged or unnecessary exposure of hair to strong alkali at about pH 12 or above can weaken, break and dissolve the hair. In some instances, strong alkali discolors the natural color of the hair. For example, the tone of natural brown hair is reddened and natural white or grey hair is undesirably yellowed and brightness is dulled. Thus, another disadvantage is delustering of the natural sheen of the hair.
The concentration of sodium hydroxide used for modern hair-straightening procedures can vary between about 1.5 to about 3.5 weight percent, depending on whether the product is a "base" or "no-base" relaxer. The terms "base relaxer" mean that the scalp and hair line must be coated with a protective oleaginous base, such as petrolatum, mineral oil and lanolin, before applying the hair relaxer. The terms "no-base relaxer" means that the scalp need not be coated with a protective base. In some cases, where the no-base hair relaxer incorporates sodium hydroxide in an oleaginous cream base, protective base frequently need only be applied to the hairline to protect the skin around the forehead, ears and neckline. No-base relaxer processes, therefore, are preferred.
Some past attempts have been made to use chemical straighteners, other than sodium hydroxide, sulfite and thioglycolates, such as mercapto-substituted compounds and various combinations of chemical treatments and heat. A discussion of these studies can be found in the books by deNavarre, The ChemistrY and Manufacture of Cosmetics, Second Edition, Vol. IV, Continental Press, Orlando, Fla. (1975), and by Sagarin, Cosmetics: Science and Technology, Second Edition, Vol. 2, Wiley-Interscience, New York, N.Y. (1972). The disclosure of both of these books are incorporated herein by reference. A review of other chemical hair straighteners and waving agents reported in the patent literature also can be found in Cosmetics & Toiletries, 94, 61-69 (April) 1979 and 100, 23-29 (April) 1985, also incorporated herein by reference. These attempts have met with varying degrees of success. But, except for possibly guanidine hydroxide, other chemical hair-straightening agents have not achieved any substantial measure of practical or commercial importance in the hair-straightening arts beyond scientific interest.
In cold hair waving arts, which by analogy extends to hair-straightening arts, some interest and success has been reported most recently from the use of the sulfur-containing amino acid, cysteine, its mineral acid salts, especially cysteine hydrochloride, and certain cysteine derivatives, such as N-acetyl-L-cysteine. Cysteine and cysteine-derived compounds are of cosmetic interest, because they are water-soluble, substantially odorless and acceptable for use in cold-wave processes as a hair keratin-disulfide reducing agent. The terms "cold-wave process" are used in the conventional sense to mean that permanent hair waving or straightening is achieved at between ambient room temperature and body temperature, usually at about 30 degrees C. (about 86 degrees F.) without the assistance of externally applied heat.
Cysteine, however, used as a main hair-straightening agent has certain disadvantages. For example, relatively high amounts of upwards of about 3 weight percent to about 20 weight percent of cysteine (or a cysteine derivative) are reportedly required to effect a beneficial change in the configuration of the hair. Like other sulfhydryl compounds, cysteine acts on the cystine disulfide bonds in hair keratin, so the treated hair must be chemically neutralized to re-link the hair keratin-disulfide bonds. In addition, cysteine oxidizes readily to insoluble cystine which can deposit as crystals on the skin and form a dulling film on the hair.
The reported useful pH of various compositions containing cysteine or cysteine derivatives in the literature generally ranges between about pH 6 to about pH 11. However, to minimize skin irritation and maximize the action of the cysteine or its derivatives on hair, a pH of about 10 or less is usually required. Sodium hydroxide has been disclosed as a cosmetically acceptable inorganic base to adjust the alkalinity of some of the foregoing compositions reported in the literature. However, volatile ammonium hydroxide and organic amine bases are generally prepared, and the amount of sodium hydroxide, if used, to adjust the pH is rather miniscule. For example, it is known that a solution of sodium hydroxide at 0.05 weight percent has a pH of 12. Thus, even if some sodium hydroxide were available as free base at a pH of about 11, the amount present would be too low to straighten hair. It is also known that sodium hydroxide at below about pH 12 and at a titratable alkalinity of less than about 0.5 weight percent effects substantially no straightening of curly hair. Thus, cysteine and other water-soluble cysteine derived compounds have not provided a viable alternative to sodium hydroxide-based hair-straightening.
Some strides have been made in improving the conditon of sodium hydroxide-straightened hair by incorporating conditioners into the alkaline product. See, for example, U.S. Pat. No. 4,175,572, which issued to our assignee and which is incorporated herein by reference. However, there is still a need for a sodium hydroxide-based hair straightening composition having relatively low concentrations of sodium hydroxide for use in a no-base relaxer process, yet retaining beneficial hair straightening effects equivalent to or better than those obtained from higher amounts presently used. An ideal sodium hydroxide-based hair straightening composition, system and method would also provide a relatively fast processing time and good hair condition.
The hair straightening compositions, system and method of this invention provide a solution to this long-standing need by providing an improved sodium hydroxide-based hair straightener containing an auxiliary hair-straightening agent in which many of the above drawbacks are overcome. In addition, the auxiliary hair-straightening agent is a polar hair keratin-disulfide reducing agent having an active sulfhydryl functional group available from the sulfur-containing amino acid, cysteine and cysteine-derived compounds. Thus, good hair-straightening benefits are retained and good hair condition is obtained.